Presently, with macOS 10.14.4, Safari 12.1, Chrome 75, and Edge Canary 77, a select element with the disabled attribute may still interacted with if using VoiceOver.

For example:

<div style="border: 1px solid;">
  <label for="t1">Set to disabled</label>
  <select disabled id="t1">

Which produces the following:

With VoiceOver on, navigate to the select and press Control + Option + Space. The option menu will be revealed and the selected option can be changed as if the element were not “dimmed.”

Investigating further, a Webkit bug had already been filed and testing again with the Safari Technology Preview v85, a fix has been implemented that resolves the issue.

However, the issue remains in Canary builds for Chrome and Edge on macOS, which makes sense since the fix will land in Webkit and not Chromium. A bug has been filed against Chromium.

There is no reasonable fix that an author could do to prevent this (very specific) issue, presently. Best to wait for a fix to land in Chromium.